Method of producing extensible paper



Aug. 11, 1970 c. B. A. IHRMAN 3,523,865

METHOD OF rnonuome EXTENSIBLE PAPER Filed March 21, 1967 Fig.1

flf'TOR/VEYS United States Patent US. Cl. 162-111 3 Claims ABSTRACT OF THE DISCLOSURE Extensible paper of improved properties, including toughness and tensile strength, is disclosed. A running paper web formed ofa suitable moisture content is mechanically shrunk in its machine direction. This web is dried while supported upon heated gaseous medium, thereby allowing the web to shrink essentially without restriction in its cross machine direction. By this process, an 8% stretch in both the cross machine direction and the machine direction of the paper may be obtained. lln a preferred embodiment, the paper Web is subjected to a draw tension in the machine direction while it is supported on the heated gaseous medium. This draw tension is in excess of the tension necessary to draw the wet web through the gaseous supported drying zone, and may actually stretch the paper, thereby removing a portion of the mechanical shrinkage previously imparted thereto. In addition to improved physical properties of the paper produced, the process allows higher machine speed, better web guidance, and freedom of cockling of the paper.

RELATED APPLICATIONS The present application is a. continuation-in-part of Ser. No. 323,055, filed Nov. 12, 1963, and entitled Method of Producing Extensible Paper, and now abandoned.

OBJECTS OF THE INVENTION An object of the invention is to produce paper which is extensible not only in one direction, the machine direction or the cross machine direction, but is extensible in both said directions.

Another object of the invention is to produce extensible paper on a paper machine having a web speed in the order of 1,500 feet per minute.

Still another object of the invention is to produce paper by a method enabling simultaneous adjustment of the extensibility values of the paper in the machine direction and in the cross machine direction to suit the requirements of the paper produced.

A further object of the invention is to enable the use of a comparatively high draw tension in the paper web when conveyed airborne through a drying section of the paper making machine, and yet retain a sufficient extensibility of the paper in the machine direction.

An additional object of this invention is to provide an extensible paper with improved physical properties, and paper bags made therefrom.

Yet another object of this invention is to provide a paper, such as kraft paper, with about 8% stretch in both the machine direction and the cross machine direction.

Still further objects and the entire scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and ice specific examples, While indicating preferred embodiments of the inveniton, are given by way of illustration only, since various changes or modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.

BACKGROUND OF THE PRIOR ART Methods of mechanical shrinking a moist paper web in the machine direction are known. For example, such methods are disclosed in the US. patent specifications Nos. 2,624,245 and 2,667,910, the French patent specifications Nos. 1,248,905 and 1,261,100, the British patent specification No. 846,646 and the Swiss patent specification No. 246,998. By these known methods, shrinkage in the machine direction can be performed to a desired extent comparatively easily, but it is diflicult to combine the same with shrinkage in the cross machine direction and to control said shrinkage. These known mechanical shrinking methods are not suited for performing shrinkage of the paper web in the cross machine direction of the web.

It is known that paper may shrink as much as 10 to 12% when supported in such a manner during its drying that no force or restraint opposes the shrinkage, e.g., when supported on a mercury bath. Such conditions are very hard to establish in commercial paper making machines, because the wet and consequently weak paper web needs support by felts or similar means. Thus, in a paper making machine drying section comprising steam-heated cylinders for evaporating the moisture of the paper web, the natural shrinkage of the web concomitant with the reduction of the moisture content from about 50% to about 15% is hindered as the Web is securely held between cylinder surface and a felt enclosing the same. Moreover, the conveying of the web through the machine requires a certain pull in the web, which also affects the rheological properties of the paper and destroys or reduces the extensibility that would otherwise accompany the natural shrinkage of the paper during its drying.

It is also known, e.g. from the Allander Canadian Pat. No. 614,598, how to perform the drying of a paper web in the interval of moisture content where the desired shrinkage takes place, by means of hot air supplied beneath the web so that the web is supported by the hot air, i.e., the web is airborne. The shrinkage in the transverse direction can then take place without restriction. However, in order to draw the web through the air drying section of the paper making machine and to guide the web in its proper path, a stretching pull must be maintained in the web. The natural shrinkage in the machine direction will be partly hindered by this pull, and in practice it is seldom possible to obtain a stretchability of the paper so produced higher than about 4%. The draw tension in the paper being dried favors shrinking in the cross machine direction, and therefore the extensibility of the paper in that direction can easily become too high. In order to get the desired cross machine shrinkage, which varies not only with the draw tension but also with variations of the properties of the pulp supplied to the paper making machine, the draw tension must be kept under very tight control in order to get a uniform paper quality. However, the practical possibilities of such tight control are very restricted. Theoretically, such an air supported drying method allows a free shrinkage of the paper web in all directions but in practice such a method involves serious drawbacks in that the web must be fed with a very low draw tension in order to allow the web to shrink in the machine direction. Such low draw tensions necessarily involve low machine speeds and/or difliculties of guiding the web. In addition, the web cockles or easily when using this prior art method of shrinking the paper.

SUMMARY OF THE INVENTION The present invention relates to a method of producing paper having good extensibility properties as well as good toughness and tensile strength, properties which are particularly desirable in the manufacture of paper bags or sacks. In particular, the invention relates to a method of treating a running web of paper in such a manner as to shrink the paper web in the machine direction as well as the cross machine direction.

This shrinkage is achieved by the combination of a mechanical shrinkage unit, which shrinks the paper web in the machine direction, with a second shrinkage unit involving removing moisture from the web, or drying the web, while the paper web is supported by a heated gaseous medium, e.g., air, carbon dioxide, nitrogen, etc., whereby shrinkage of the web in its cross machine direction is obtained.

The process to the present invention eliminates difiiculties encountered in the extensible paper processes of the prior art and allows the production of extensible paper with the desired degree of extensibility in the machine direction as well as the cross machine direction, at high machine speeds.

By combining mechanical shrinkage in the machine direction of the web with a free shrinkage taking place in connection with the drying of the web while supported by a heated gaseous medium, stretching the web in the machine direction is less critical and consequently it is easy to guide the web through the air drying section, so that a high speed thereof can be maintained, and by varying the draw tension of the rheological qualities of the paper can be varied over a wide range and adjustment to varying operational conditions is easily and quickly made.

According to the invention, a running paper web formed in any conventional way and with a suitable moisture content is first shrunk mechanically in its machine direction. Thereafter the running web is dried under conditions allowing the web to shrink unrestrictedly or essentially unrestrictedly in its cross machine direction. This drying may take place while the paper web is airborne, as according to the known gas supported drying method, or it may be dried in any other manner fulfilling the requirement of essentially free cross machine shrinkage of the web. During this drying the web is subjected to a draw tension in the machine direction. This tension may be chosen so that the web can be run with an increased speed, as compared to known driers using an airborne web. Moreover, on account of this higher tension, the guidance of the web will involve no problem. The draw tension is so adjusted that the paper web, instead of being allowed to shrink to a further essential extent, is allowed to shrink only moderately in its machine direction during the drying, or is even partly stretched out anew, while the magnitude of the cross machine shrinkage is not restricted. The value of the stretching tension used may depend upon the properties of the fiber material used and the properties desired in the final product. Moreover, with regard to the desired paper properties, the draw tension may be adapted to the different moisture contents of the web during the drying, i.e., the draw tension may be varied stepwise in different zones, or levels, of the drying as well as before and after the same. The different zones may be separated in a suitable manner, such as by presses. The draw tension can be measured by means of torque feeling means coupled to the shafts of the presses, or estimated from the differential speeds of the presses.

DESCRIPTION OF THE DRAWINGS The processes of the present invention will be more fully understood with reference to the accompanying d drawings which illustrate a paper making machine upon which the present process may be performed.

FIG. 1 is a block diagram of the paper making machine;

FIG. 2 schematically illustrates the air drying section and the shrinkage unit of the machine in FIG. 1;

FIG. 3 is a vertical elevational enlarged view of a portion of the apparatus of FIG. 2;

FIG. 4 is a top plan view of a portion of a blow box which forms part of the air drying section of FIGS. 2 and 3; and

FIG. 5 is a vertical cross section of the blow box of FIG. 4, taken along the line V-V.

In FIG. 1, A designates the wire forming and wet press section of a paper making machine, which section may be of any conventional design. A pulp slurry, for example, a slurry containing 0.5% of a cellulosic fiber material and 99.5% of water, is supplied to this section, and a running paper web is formed therein and dewatered to a moisture content of, for example, about B designates a predrying section, comprising a series of conventional steam heated rolls, against which the delicate paper web is held in contact so that a part of its water content is evaporated. The web leaves this section with a moisture content of, for example, about 50% and enters the next section, e.g., the air-drying section B in which the Shrinking unit may be incorporated and which is shown in more detail in FIG. 2. With a moisture content of, for example, about 15%, the web then runs into the final drying section B comprising a series of conventional steam heated rolls and in which the moisture content of the Web is reduced from, for example, about 15% to a value of 5 to 7%, i.e., the air-dry value at which the moisture is at equilibrium and the web neither absorbs moisture from nor gives ofi? moisture to dry atmospheric air. By performing the final drying with the paper being supported upon the surface of cylinders, cockling will be prevented. The paper web is then calendered in section C and rolled up.

As shown in FIGS. 2 to 5 the air-drying section B comprises a number of blow boxes 1 arranged in, for example, five tiers, each tier consisting of horizontal parallel boxes located at the same level and having a width, measured in the cross machine direction of the paper making machine, essentially corresponding to the width of the paper web. Heated air or other heated gaseous medium is supplied to both ends of each blow box and is let out as jets 2 preferably directed alternately in the forward and backward direction, as related to the running direction of the web, through eyelid openings 3 provided in the top wall of the blow box and evenly distributed over its length. The web w is led back and forth over the tiers of blow boxes in five superimposed passes or runs 11, 12, 13, 14 and 15 separated by turn- 1ng rolls 16, 17, 18, 19 located at opposite ends of the drying section B The air jets form cushions of air upon which the paper web floats freely, or is supported, so that its weight is counterbalanced and need not be supported by the turning rolls-in other words, the web is airborne. Due to interaction between the static and the dynamic air pressure upon the web, the paper web will be held at a constant distance, determined essentially by the velocity of the air, from the tops of the blow boxes. An overpressure in the boxes, for example, of the order 1" Water column, has proven to give stable running conditions. There is only a very slight friction between the web and the air cushions and therefore the paper web can be advanced with a very small force in its machine direction. Such a force, which is called the draw tension, is insignificant in comparison to the forces in the paper striving to contract or shrink the paper as it dries on account of evaporation of part of its moisture content caused by the heating by the air jets. In the cross machine direction the web is fully or almost fully unrestricted to shrink, but in the machine direction the shrinkage is restricted more or less according to the draw force or tension applied. This draw force is preferably chosen above the value required for merely overcoming the friction opposing the movement of the web, for the purpose of maintaining the web smoothly extended and properly guided, so that it does not drift sidewise.

While flowing along the under side of the web the heated air absorbs moisture therefrom. The air departs downwardly through the spaces between the blow boxes and then flows horizontally in the cross machine direction of the machine while absorbing some moisture from the upper side of the next underlying run of the web. At opposite sides of the machine the used drying air is sucked out by fans and sent through heating batteries and then recirculated to the blow boxes for renewed use. Wet air is bled off the circulation, and dry air is admixed in order to maintain a sufficiently low moisture content of the circulating air. The temperature of the air ejected from the blow boxes is preferably held between about 100 to about 120 degrees centrigrade and the wet temperature thereof may be, for example, about 60 degrees. By controlling these temperatures the total amount of evaporated moisture, or the drying capacity of section B can be controlled and brought to suit varying working conditions.

In a paper making machine with a designed web speed of about 1500 feet per minute, for a 200 inch wide web, there are required about 600 blow boxes in the air drying section, which correspond to a freely suspended Web length of about 450 feet (each blow box is, for example, about 6 inches in size, and has a spacing from the next iblow box of 4 inches). The arrangements of the blow boxes in five superimposed tiers reduces the length of section B to about 90 feet, and its height may be about 4 feet, for example.

In addition to the shrinkage in the cross machine as well as the machine direction of the web caused by drying while airborne, the paper web is also subjected to a mechanical shrinkage in its machine direction while having a moisture content preferably within the range of 28 to 42%, and most preferably about 30 to 40% moisture content. This mechanical shrinking is performed by a unit shown in the left part of FIG. 2, comprising a pair of rolls 21, 23. Roll 21 is heated and preferably has a steel surface, and roll 23 is rubber covered. The rolls are pressed together and driven at differential speeds, the rubber roll running slower.

A moist paper web passing the pressure nip of rolls 21, 23, will be compacted in its machine direction to an extent depending upon the line pressure of the nip and the speed differential of the rolls, whereas the width of the paper web is left substantially unaffected. The details of such a shrinking or compacting unit and its operation may correspond, for example, to French Pat. No. 1,248,905 or to US. application Ser. No. 383,170.

A paper web having a moisture content of between 28 and 42% can be mechanically shrunk in said unit, although the best results are obtained when the moisture is about 30 to 40%. With five runs of the paper web back and forth through the air drying section for a designed total drop in moisture content from 50 to 15%, a suitable moisture content of about 35% is obtained after the second run 12, and therefore the web need not turn about the roll 17, which may be removed, but is drawn through the nip of the compacting unit 21, 23 and then returned to the air drying section in order to make its third run therethrough.

The turning rolls 16', 17, 19 are idling and essentially do not change the tension present in the web passing thereover. The turning roll 18 at the end of the third run of the paper web may cooperate with a driven roll 25 or even a nip with one driven roll, and the web may be drawn out of the section B by a driven roll 27. By controlling the speed of the rolls 25, 27 in comparison the speed of the compacting unit a desired tension can be maintained in the web, whereby the shrinkage of the web in the machine direction caused by the drying is partly or wholly eliminated and also a part of the mechanical shrinkage performed by the shrinking unit 21, 23 can be removed.

In order to prevent wrinkles and to straighten out the paper web in the cross machine direction at least some of the turning rolls may be replaced by bent rolls of a kind known to the art under the registered trademarks Mont-Hope or Varibow. Such a roll 29 is shown in FIG. 2 close to the turning roll 19, which it may replace. Another bent roll 31 is placed immediately before the compacting unit. These bent rolls may also effect a certain stretching of the web in the cross machine direction, whereby part of the transverse shrinkage caused by the drying of the web while airborne might be eliminated.

DETAILED DESCRIPTION OF THE INVENTION The modified paper making machine which was described in the section entitled Description of the Drawings may be used in the process of the present invention. The numerical values mentioned in the description of the drawings were merely set forth as examples for a clear illustration of the invention and are not to be construed as limitations thereof. The process of the present invention may vary over wide ranges according to various operational factors and the quality of the papers desired. The shrinkage performed by the compacting unit may vary from 1 to 12%, preferably about 3-4%. It should be observed that in order to obtain a paper with 8% extensibility in the machine direction, the mechanical compaction need not be as high as 8% in spite of the subsequent partial removal of the mechanical shrinkage so imparted. This is in distinct contrast to the prior art processes wherein, in attempting to produce paper with a comparable degree of longitudinal extensibility, a mechanical compaction of 8% or more is required, which involves a significant speed differential of the compacting rolls with a consequential high degree of wear of the rubber surface and also a considerable power consumption.

When higher mechanical compaction values are used, e.g. 4-6% mechanical shrinkage, a higher removal of this mechanical shrinkage can be used in the air-supported drying section, thereby favoring transverse shrinkage. A transverse shrinkage of too high a value may be controlled or counteracted by use of the bent rolls 29-31.

The tension in the various runs of the paper web may be varied from a value barely high enough to keep the web running in an orderly manner to a value corresponding to the complete elimination of the shrinkage effected by the mechanical compaction unit and sometimes even elimination of part of the natural shrinkage .having taken place in other parts of the drying section where the draw tension is so low as to allow shrinkage of the web in the machine direction. Thus, the extent to which the mechanical shrinkage is subsequently nullified by the draw tension applied to the webduring its travel through the air drying section may correspond to an elongation of, for example 1 to 4%. This elongation may take place either mainly during the first horizontal run of the paper web following the mechanical compaction, which seems to increase the yield strength of the paper and is preferred, or the elongation may be distributed more or less evenly over all of the subsequent runs of the paper web, the latter alternative requiring control of the speed of additional turning rolls.

Generally, the draw tension applied to the web subsequent to the mechanical compaction step will be measured by means of a driven roll or rolls (such as roll 25) which are turning at speeds up to 5% faster than. the paper web leaving the mechanical compacting unit. The paper making machine may be operated over a wide range of speeds including quite high speeds, such as 1500 ft./ min. or higher.

In some cases where a high mechanical shrinkage is desired, the same can be divided up into two steps and be performed by two compactors working upon the paper web at points having different moisture contents within the above-mentioned range. Thus in FIG. 2, the rolls 18, may be replaced by a second compacting unit, for instance of the same kind as the unit 21, 23. In this case the two compactors can be driven with a lower speed difference of the rolls than with a single compactor, whereby the wear and the power consumption are reduced and regrinding the rubber roll surface is not required so often. The number of blow box tiers may also be increased in which case it is easier to arrange the compactor or compactors at points where the paper web has the moisture content best suited for mechanical compaction. Alternatively the longitudinal shrinkage may be performed by machinery of another kind than that mentioned above, e.g. by means of a compactor comprising an extensible rubber blanket as shown in US. Pat. 2,624,245. By using a compactor of the type disclosed in the US. application Ser. No. 383,170, the moisture limit for mechanical compaction may be extended to a moisture content of 50%. For restricting the transverse shrinkage and straightening the web in the cross machine direction a curved stationary rod can be substituted for the bent roll 31 or 29, especially when a comparatively heavy paper is treated.

While the process described with reference to the accompanying drawing is the preferred embodiment of the present invention, other embodiments of the process for producing extensible paper may be utilized. For example, the predrying section B using a series of conventional steam heated rolls, may be omitted and the paper web is thereby introduced into air drying section B with a moisture content of about 70%. If necessary, additional tiers of blow boxes may be incorporated into air drying section B in order to handle the increased drying, or moisture removal, requirements.

Another modification of the extensible paper process may be in placing the mechanical shrinking unite.g., rolls 21, 23-between drying sections B and B In such a modification, the cylinder drying section (pre-drying section) B reduces the moisture content of the paper web from about 70% to about 42 to 28% moisture content. The present invention contemplates mechanically shrinking the paper web either before or after the paper has been transversely shrunk while supported on a heated gaseous medium, or the mechanical shrinking step may be intermediate, or during, cross machine shrinkage, as illustrated in FIG. 2.

Shrinking the web mechanically as described in the Cluett US. Pat. 2,624,245 should be at a moisture content not higher than about Using the method disclosed in my copending US. application Ser. No. 383,170, now U.S. Pat. 3,290,209, it became possible to increase this moisture content region of mechanical compaction to about moisture content. If the paper is compacted at a moisture content considerably below 30% the surface of the paper will show creases and become uneven, which makes it less suited for printing. Thus, the mechanical shrinking of the paper is performed in the region of 30-50% moisture content of the paper web.

Because of the high tendency of the wet web to shrink when its moisture content is in the range of 15-50%, it should be essentially free to shrink in the cross machine direction, if possible, in this drying interval.

For the production of a paper with the desired stretch, e.g., 8% in both directions of the web, the machine set up could be somewhat as follows. Starting from the wet web an ordinary paper machine can be used until the part in the drying section where the web is dried to 50%. Thereafter the web could be dried in the air-supported way disclosed by Cluett or with a double roll. After that a drying section according to Allander could be used, although the Allander process, however, is not a limitation but any method that allows the paper to shrink freely in the cross direction under tension control according to this invention could be used. To eliminate a tendency of cockling ordinary drying cylinders should be used when the web reaches a moisture content of about 15%. From there on to equilibrium with the surrounding air, a moisture content of 5-7%, ordinary drying cylinders can preferably be used. After that the paper Web is reeled in an ordinary way, and calendering or even size pressing the sheet can be performed at a suitable known moisture content.

On trying to run a machine unit consisting of an ordinary paper machine and apparatus for mechanical shrinking and free drying, one obviously would expect troubles on guiding the mechanically compacted paper through the drying section where the paper should shrink freely. If the web is not supported by jets of a gaseous medium, e.g., hot air, the weak web would exhibit severe crinkles and creases in any direction. This would require significant speed reduction of the machine and be commercially unfeasible.

It is known that on compacting paper with blankets and rolls covered with soft materials like rubber, a certain loss of rubber will take place. Using a rubber roll with a diameter of 600 mm. the roll has to be reground after about 200 hours of producing a paper with about 8% machine direction stretch. With the combination of mechanical shrinking and free drying, the necessary mechanical compaction in the machine direction need amount to only about 4%. This would obviously be a gain, reducing the need for grinding. However, according to the present invention, the most suitable way of producing a paper in a controlled way is to shrink the paper more than needed and during the free drying of the paper pull some of this shrinkage out. The most suitable embodiment is to have several drying sections stacked over each other and at the turning points control the draw in the web.

Ordinarily draw tensions are very hard to measure, and the workman in the art often refers to the relative speed of the turning rolls. In the section where the paper is allowed to dry freely, the workman would expect a difference in speed between the first and last turning roll of about 4%, with the last roll running slower than the first. If, however, the speed difference between the first and last rolls in this drying section is kept below 4%, a certain tension will be maintained in the web, and it has been found that this will avoid the formation of creases and crinkles in the web. This draw tension will increase when the difference in speed between the first and last turning roll is decreased. It is even possible to run the last roll at a higher speed than the first roll in the drying section, whereby, the paper is stressed and if it were not mechanically shrunk it would have a high tendency to break. For the control of this desirable tension, the apparatus may be modified so that at every turning point facilities are installed to control the speed of the turning rolls. An arrangement most suitable according to the present invention is a running of the web through the first part or deck of the drying section, where the paper can shrink freely in such a way that the entering speed is much lower, e.g., 1-3% lower, than the outgoing paper speed. Clearly, the paper will be stretched during this part of the drying. It was found that the mechanically shrinking of the web and the immediately afterwards pulling up to 2% of this forced shrinking out produced the paper with a higher yielding point in the rheological sense compared to similar paper where the pulling out was spread over the whole drying section. It was also found that only one deck of the drying section was less suitable than at least 3 decks, e.g., 5 decks. For controlling the cross machine stretch, which on some occasions might be too high, it was found suitable to guide the web to or from the turning rolls via certain types of rolls which can be or are bent. Suitable types of such bent rolls, which are well known to the art, that can be used for a stretching or widening of a web, are those sold under the registered trade marks Mont-Hope and Varibow although other makes can also be used. If the paper is held under tension, the cross machine stretch can be controlled to a high degree, whereby the bent rolls give an additional improvement. On heavy papers, as above 100 g./m. even curved rods might be used for the web widening.

One of the outstanding characteristics of the present invention is the elimination of the disadvantages of the previous air-supported shrinking processes, such as the Allander process. A very low tension in the running direction of the web in order to get a high stretch in that direction is unnecessary when the method of the present invention is used. If no need exists to produce a paper with about 8% machine directional stretch, e.g., Clupak paper, a modification of the present invention may be used. Double roll compactors constructed to produce a paper with more than 8% stretch in the machine direction are expensive to use due to the high power requirement. Power is fed to the steel roll of the compactor, and some of the power is taken out from a breaking generator connected with the rubber roll and fed back to the motor driving the steel roll. A power circulation is thus built up. The expensive parts are the motors and the generators together with very heavy gear boxes. The power needed might on some cases run up to 1,000 PS. If only a very small compaction, e.g., about 1 or 2%, is desired, one might instead use simple mechanical devices, such as straight connections via gear boxes, between the steel and rubber roll and the power circulation might only be about 100 PS, for example. A double roll compactor constructed this way could thus be used as the turning rolls on each deck of the gas-supported drying section. The compaction is controlled by the pressure between the rubber roll and the steel roll. The rubber roll surface could be run 3% slower than the steel roll surface, for example, and if the pressure between the rolls is about 15 kg./cm. length, a compaction of about 3% is to be expected, if the moisture content of the sheet is in the region of 30-40%. Such a compactor could be used at the end of the first deck in an air-supported drying section, whereby two decks in such a section could be connected. In the drying deck after the compactor a pulling force is desired. At the end of the second section another compactor may be placed and the speed of this compactor compared to the first compactor acts as the controlling element of the degree of machine directional stretch. The second compactor can be used to pull out the compaction that the first compactor generates. A press of this kind might then be used at the end of every gassupported drying section. 'It is, however, not necessary to have a compactor at the end of a section, as a size press could also be used, and in some cases a breaker or a smoothing press or even a calender stack might be appropriate. Using the present invention in this way the runability of the gas-supported shrinkage process is considerably increased by the use of the tension control to pull out stretch in a mechanically shrunken paper.

The described apparatus illustrates some typical use of the invention. Through a combination of mechanical shrinking of a running wet paper with a drying of the web while supported on heated gaseous medium in a manner using tension control of the web in the drying section, the advantages of both types of processes are maintained but the drawbacks of the processes at the same time will be eliminated. The speed of the turning rolls to control the draw tension is preferred, but other means for sensing the draw tension, like torque feeling equipment or driving power, might be used.

Although impregnants of a resinous or polymeric nature could be used in papers which are treated by the process of the present invention, such impregnated papers are not suitable for paper bag production, from both a physical property and an economic standpoint; therefore, the preferred embodiment of the present'invention contemplates the treatment of paper which does not contain a unifying agent, such as various extensible polymeric materials, or a fixing agent, such as a resin. Also, it is preferred that the paper be relatively smooth, e.g., not embossed. Thus the present process can be readily distinguished over the prior art processes which use impregnants to produce products which are unsuitable for making paper bags. These prior art processes are also distinguished from the present invention in that such prior art impregnated paper processes use a process involving drying a paper web by means of heat supplied as heated air. In distinction, the resent process uses jets or cushions of heated air to actually support the paper web while the web is shrinking transversely. The jets of air or other gaseous medium actually form the major physical support for the paper web during the drying step involved.

EXAMPLES OF THE INVENTION The invention will be understood more readily by reference to the following examples; however, these examples are intended to illustrate the invention and are not to be construed to limit the scope of the invention.

The apparatus described in the section entitled Description of the Drawings was used in the following examples:

Example 1 A paper web was marked in a given area to produce a paper piece measuring 100 units long and 100 units wide, when leaving the former and wet press A. During runs 11 and 12 through air drying section B the paper web was free to shrink in the cross machine direction and was subjected to a low draw tension whereby the web was not too restricted in the machine direction. Just before entering the nip in the compacting unit 21, 23, the marked paper web area measured 98 units long and 97 units wide. The compacting unit heating nip pressure was about lbs. per linear inch of the nip and the speed diiferential of rolls 21, 23 was of about 3%, with the rubber coated roll 23 being the slower roll. With these conditions the Web was subjected to a compaction in the machine direction of about 3%, causing the marked paper web area to be about units by 97 units when leaving the compaction unit. The roll 25 was driven with a speed which was 2% higher than the speed of the paper Web leaving the compactor, and therefore in the third run (13) of the paper web through the air drying section B the tendency of the web to shrink in the machine direction because of the drying or moisture removal was prevented. In fact, a greater part (about of the shrinkage previously parted to the web in the compactor was removed. Because of the draw tension thus maintained on the paper web the shrinkage of the web in the cross machine direction actually exceeded the value which would be obtained when no such draw tension was present. Because of this, when passing roll 18 the previously mentioned marked paper web area was 97 units by 94 units. Roll 27 was driven with a somewhat lower speed than roll 25 (about 5% lower) with the paper being allowed to shrink naturally in the 4th and 5th runs (14 and 15), even in the machine direction. However, a sufiicient tension was maintained to hold the paper web flattened out and to allow proper guidance of the web. The marked paper web area reached the air dry section B with a length of 92 units and a width of 92 units and maintained these measurements during the subsequent final drying operation. The paper thus obtained had an extensibility of about 8% in either of two directions of right angles with each other and therefore was highly useful for the production of paper bags or sacks. The paper was also made comparatively stilt as opposed to the paper produced by prior art process, and this stiffness favored the handling of the paper in bag making machines.

Examples 2-9 (comparison with the prior art processes) In the following Examples 2-9, Examples 2-5 relate to a sulphate paper produced using four diiferent processes 1 l and Examples 6-9 refer to sulphite paper manufactured in the same four different processes.

Example 2 A sulphate paper was manufactured in the conventional way without mechanical shrinkage and in a paper making machine merely having cylinder driers.

Example 3 A sulphate paper was manufactured from the same pulp as in Example 2 Without mechanical shrinkage and in a paper making machine having a drying section in which the paper web was airborne.

Example 4 A sulphate paper was manufactured from the same pulp as in Example 2 and in the same machine as there, but with a shrinkage unit inserted in the early part of the cylinder drying section of the machine and comprising a pair of cooperating rolls, one roll being rubber covered and driven at a 6-8% lower speed than the other hardsurfaced roll, whereby about 68% mechanical shrinkage was imparted to the paper.

Example 5 A sulphate paper was manufactured from the same pulp as in Example 2 in a paper machine of the kind previously described with reference to FIGS. 1 to 5 and comprising not only a shrinkage unit but also a drying section in which the web was air-borne. The paper was mechanically shrunk 68%, in the machine direction. The drying section corresponded to that illustrated in FIG. 2, with the paper web dried to 35% moisture content entering the mechanical shrinking unit.

Examples 6 to 9 A sulphite paper was treated as according to the above Examples 2 to 5. However, the paper had a somewhat higher area weight, 90 grams per square meter, as compared with 70 grams per square meter of the sulphate paper. The process of treating paper in Examples 6-9 corresponded to Examples 2-5, respectively.

The papers obtained from the above Examples 2-9 were tested for tensile strength, ultimate elongation and fracture energy. The results obtained for both machine and cross machine direction are reported in the following table:

Ultimate Fracture energy, Ten ile strength, elongation, kilogram-force, kilogram-force percent centimeter Example 1 t 1 t 1 t.

llongitudina1 (machine), ttransverse (cross machine).

The above examples show the improvement of the paper qualities made possible by the use of the process of the present invention. Also important are the advantages in the adaptability of this process to various kinds of paper and to varying operation conditions as Well as 12 in the steady web guidance, the high speed and the freedom of cockling which are obtained.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

I claim:

1. A method of treating a running wet paper Web during a paper manufacturing process, said process comprising the steps of mechanically shrinking the paper web about 1 to about 12% in its machine direction while the web contains 30 to 50% by weight of moisture, and thereafter drying the paper web by passing through a heating zone wherein the moisture content of the web is reduced to a moisture content of at least 15% by weight while the web is supported in said heating zone upon a heated gaseous medium, and whereby said web shrinks in its cross-machine direction, stretching the running paper web about 1 to about 4% in its machine direction while in said heating zone to remove part of the mechanical shrinkage previously imparted to the web.

2. A process for treating a wet paper web during a paper manufacturing process to improve the extensibility of the paper product, said process comprising the steps of mechanically compressing a moist paper web about 3% in its machine direction, said paper web having a moisture content of about 30 to about 40% by weight, and thereafter shrinking the moist paper web in its cross-machine direction by removing moisture from the Web by drying to a moisture content of at least about 15 while the web is supported by heated gaseous medium and essentially unrestrained against cross-machine directional movement, whereby unrestricted shrinkage of the web in its crosmachine direction is obtained, and stretching the mechanically shrunk web by about 2% of its length.

3. A process for treating a wet paper web during a paper manufacturing process to improve the extensibility of the paper product, said process comprising the steps of mechanically compressing a moist paper web about 3% of its length while at a moisture content of about 30 to about 40% by weight, thereafter drying the paper Web, with removal of moisture, while supported on heated gaseous medium, stretching the paper web to remove about two thirds of the mechanical shrinkage previously imparted thereto, and then further drying the web by removing moisture from the web by drying to a moisture content of at least about 15% while the web is supported by heated gaseous medium and essentially unrestrained against cross-machine directional movement, whereby unrestricted shrinkage of the web in its cross-machine direction is obtained.

References Cited UNITED STATES PATENTS 2,825,117 3/1958 Evans et a1. 162-206 3,055,496 9/1962 Dunlap 162206 FOREIGN PATENTS 614,598 2/1961 Canada.

HOWARD R. CAINE, Primary Examiner US. Cl. X.R.

33 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,5 3, 5 Dated Au ust 11. 1970 Inventor(s) Carl Bertil Andreas Ihrman It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Parent application is reflected as being Ser. No. 323,055, No. 26, 1963. This should have read Ser. No. 323,055, Nov. 12, 1963.

NOV. 17,1970

Edmdlml r JR. L 0mm Oomissiom of num 

